What is the authenticity of the hadith: "Indeed, a servant can reach by virtue of his good character the rank of one who fasts and prays at night," and what is the ruling on one who says: "It is permissible for a disbeliever to remain in his religion on condition that he believes that Muhammad (peace be upon him) is a messenger from Allah"?

Good character elevates a believer to the rank of one who fasts and prays through the night, whereas a non-Muslim's good character will not benefit him in the Hereafter, because there is no salvation except by following the Law of Muhammad, peace and blessings be upon him. And whoever believes it permissible to deviate from it has committed disbelief.